It is known practice to dye keratin fibers, for instance human keratin fibers such as the hair, with dye compositions comprising direct dyes. These compounds are colored and coloring molecules that have a certain affinity for the fibers. It is, for example, known practice to use direct dyes of nitrobenzene, anthraquinone, nitropyridine, azo, xanthene, acridine, azine or triarylmethane type or else aromatic direct dyes comprising a hydrazone function.
Unlike the colorings obtained using oxidation dye precursors which develop the color actually within the fiber, by means of a condensation process in an alkaline oxidizing medium, direct dyes do not penetrate deeply into the keratin fiber, but remain located rather at the surface or in the fiber close to the surface.
Thus, the colorings that result from the use of direct dyes are temporary or semi-permanent colorings since the nature of the interactions which bond the direct dyes to the keratin fiber, and their desorption from the surface and/or from the core of the fiber, are responsible for their poor relative fastness with respect to repeated shampooing, for example. Moreover, these dye-fiber interactions may cause difficulties during use, for instance with regard to the uptake of the dye in the fiber, which can result in insufficiently homogeneous colorings and/or in changes in coloring.